backup your old /etc/X11/xorg.conf
Now, copy and paste the section below: Adopt it to your file suitably.
Section "InputDevice"
Identifier "Mouse0"
Driver "synaptics"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/input/mice"
Option "ZAxisMapping" "4 5"
Option "Emulate3Buttons" "yes"
Option "LeftEdge" "1900"
Option "RightEdge" "5400"
Option "TopEdge" "1900"
Option "BottomEdge" "4000"
Option "FingerLow" "25"
Option "FingerHigh" "30"
Option "MaxTapTime" "180"
Option "MaxTapMove" "220"
Option "VertScrollDelta" "100"
Option "MinSpeed" "0.02"
Option "MaxSpeed" "0.18"
Option "AccelFactor" "0.0010"
# Option "SHMConfig" "on"
EndSection
Alternatively,
http://qsynaptics.sourceforge.net/
http://kde-apps.org/content/show.php?content=17286
Try these.